Described below is an attenuating mass for an ultrasonic sensor and the use of the attenuating mass.
All kinds of measuring methods exist for measuring the fill level in fluids, each having specific advantages and disadvantages. A robust and versatile measuring method involves measuring using ultrasound, in which the run time of an ultrasonic pulse is measured from the emitter to a boundary surface (e.g. the boundary surface fluid-air) and back to a receiver and the course is calculated from the known or currently determined sound velocity in the medium.
In many instances the same element generating the ultrasound, in most cases a piezoelectric converter, is used both as a transmitter and also as a receiver. The course which can be minimally measured with such a sensor (also known as blocking distance) is determined by how quickly the transmit and receive element comes to rest again after emitting the measuring pulses, so that the echo signal can be clearly detected.
This fading time is influenced by two main factors: on the one hand the acoustic coupling to the measuring medium, on the other hand the mechanical attenuation of the element. A good coupling to the medium shortens the fading time such that a large part of the sound energy can be radiated and does not have to be dissipated in the transmit element by inner friction or other loss mechanisms. Mechanical attenuation of the element destroys or dispels the residual energy in the attenuating material, so that the element itself comes to rest more quickly. It should be noted here that excessive mechanical attenuation can also negatively affect the signal amplitude and the sensitivity of the sound detection.
When used in vehicles, particularly when measuring the oil level in the oil pan of a combustion engine, it is in most cases requested that the blocking distance and thus the minimal detectable oil level be kept as low as possible. To this end, it is necessary to significantly attenuate the fading time of the transmit and receive element, wherein this attenuation has to function across a very wide temperature range.
Interfering signals which are produced from a reflection on the rear side of the sensor, develop due to the pulse/echo method introduced, particularly in the event of inadequate attenuation. In order to suppress these unwanted signals, the rear side of the ultrasonic source is provided with an attenuating mass. Casting compounds which are filled into the plastic housing are used here.
DE 3431741 A1 discloses an apparatus and a method for measuring the fill level of liquids, wherein in closed containers, an ultrasonic sensor which is applied from the outside is coupled in a planar fashion to the flat or curved container base by way of a medium. An epoxy resin adhesive may be used as a medium.
No casting compounds were however known up to now which indicate the required ultrasonic attenuation above a required temperature range of −40° C. to 180° C.